carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules.
an ionic compound consists of a metal and a non-metal.
oxygen and fluorine are non-metals.
sodium and aluminium are non-metals.
calcium is a metal and chlorine is a non-metal.
nitrogen and sulfur are non-metals.
thus, the answer is c.